How to create Realistic Skies with POV-Ray

Simple clear blue summer sky:

The word inside a hemisphere of type "cheese cover" :
with a realistic fog at the horizon made by "color_map{gradient <0,1,0> ..."
changing the color from white at the horizon level to deep blue at 12 am.
All together blown up by "scale 10000" - the only problem:
You must keep the sun in this hemisphere, otherwise you see only
the ambient component of all textures!

With the word inside a hemispere type "cheese cover"
you should not change to a cloud texture because of the very unrealistic clouds crashing
with the horizon!

Unfortunately also most cloudy skies created with "sky_sphere" are
suffering from this illness!

Partly cloudy sky with realistic horizon:

The sky is formed by a blue plane which is decorated by some
clouds. The clouds become realisticly smaller at the horizon!
This effect cannot be simulated by the use of "sky_sphere"!
The realistic ground fog covers the black line at the horizon - the gap between
the ground plane and the sky plane.
The only problem here:
You must keep the sun below this sky plane! - Otherwise you see only
the ambient component of all textures!
A version of a realistic cloudy sky without using fog can be found in the
2nd example at 5.